In a letter from Imam Hasan Askari to Ali ibn Babawayh al-Qummi, it is mentioned that the Messenger of Allah has said: the best deeds is to wait for the reappearance of Imam Mahdi (a.s), and my Shias will be constantly in grief until the time when my son would fill the earth with justice and equity after it would be fraught with injustice and oppression.
source : erfan.ir
